class SerialNumberGenerator{
	private static volatile int i=0;
	//不是同步的，多个线程操作可能会冲突
	public static int nextSerialNumber(){
		return i++;
	}
}
class CircularSet{
	private int[] array;
	private int len;
	private int index=0;
	public CircularSet(int size){
		this.len=size;
		array=new int[len];
		for(int i=0;i<len;i++){
			array[i]=-1;
		}
	}
	public synchronized void add(int n){
		array[index]=n;
		index=++index%len;
	}
	public synchronized boolean contains(int n){
		for(int i=0;i<len;i++){
			if(array[i]==n){
				return true;
			}
		}
		return false;
	}
}
public class Test {
	private static final int SIZE=100;
	private static CircularSet mCircularSet=new CircularSet(1000);
	private static ExecutorService mExecutorService=Executors.newCachedThreadPool();
	public static void main(String[] args) throws InterruptedException {
		for(int i=0;i<SIZE;i++){
			mExecutorService.execute(new SerialChecker());
		}
		TimeUnit.SECONDS.sleep(5);
	}
	static class SerialChecker implements Runnable{
		@Override
		public void run() {
			while(true){
				int serial=SerialNumberGenerator.nextSerialNumber();
				if(mCircularSet.contains(serial)){
					System.out.println("Duplicate:"+serial);
					System.exit(0);
				}
				mCircularSet.add(serial);
			}
		}
	}
}